Abstract
Summary The objective of this study was to determine whether progestin (norgestomet) supplementation and double-timed artificial insemination (TAI) during the Ovsynch protocol enhances fertility in dairy cows and heifers when the state of oestrus was not determined. In group I (Ovsynch), cows (269), heifers (152) were treated with GnRH on day 0, PGF2α on day 7, GnRH on day 9. TAI was performed approximately 16 h after the second GnRH. In group II (Ovsynch, norgestomet), cows (268) and heifers (152) were treated as in group I, but the implant was received at the time of first GnRH. All implants were removed at the time of the PGF2α. Cows (268) and heifers (150) in group III were treated as in Group II, but a double TAI was performed. First TAI was administrated at the time of the second GnRH and the second TAI was administered approximately 18-20 h later. Cows (270) and heifers (153) detected as being in natural oestrus were used as untreated control animals. The use of the norgestomet implant at the same time as the Ovsynch protocol did not increase pregnancy rates in cows and heifers that were classified as cyclic or noncyclic before treatments began. Total pregnancy rates were significantly greater for group III heifers than for group I, II, control heifers and group III cows.
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